Asynchronous broadband networks using packet-switched technology, such as the Ethernet, are extremely robust and reliable for the transmission of data. This robustness is in part a feature of asynchronous operation, because each node in the network is able to operate independently of other nodes and consequently high performance timing components for communication between nodes are not required. The increasing availability of these broadband networks has provoked interest in using such networks to transfer applications with strict timing requirements, such as voice or video. Mobile telephony in particular is a likely candidate in view of the widespread availability of these types of networks. However, these real-time applications impose low round-trip delays on the network. For example in voice communication, round trip delays in excess of around 100 ms are disconcerting to the user, as any delay is perceived as a pause in the conversation. Synchronous operation is also essential, for example to ensure that voice and image components of video data arrive together and also so that radio base stations are able to transmit data simultaneously.
Present day solutions to this problem include installing high performance oscillators serving as clock signal generators in the access nodes and synchronising these oscillators with an external clock source, for example using an IP-based timing protocol such as NTP (Network Timing Protocol). However, in addition to the expense required in modifying the access nodes, the time required to synchronise end nodes to a master node network start-up time and reset or failure recovery is very long when NTP is used, typically of the order of several hours if not days. In addition, the attendant increase in complexity and sophistication of the access nodes inherently changes the nature of the packet-switched network and makes it more susceptible to breakdown.
There is thus a need for enabling the reliable transmission of real-time or time-sensitive applications over existing packet switched networks that obviates the extensive modifications in the network infrastructure and enables rapid network start-up and recovery times.